(a) Field of the invention
The present invention relates to a focal plane shutter for cameras of a type wherein said shutter is equipped with a front blade group and a rear blade group, and adapted to permit double light interruption with these blade groups.
(b) Description of the prior art
In a general focal plane shutter of the above-mentioned type which is equipped with the front blade group and the rear blade group, light interruption onto the film surface is performed by covering an exposure aperture ordinarily with the front blade group in the shutter cocked state and with the rear blade group in the shutter uncocked state. Further, for the purpose of improving light interruption property and enhancing resistance to external stress, it is practiced to perform the so-called double light interruption, for which the exposure aperture is covered both with the front blade group and the rear blade group in the shutter cocked state. Especially in cases of cameras which have built-in film winders and are often left in the shutter cocked state, the necessity for the double light interruption becomes higher. In the conventional operating sequence for performing the double light interruption in the shutter cocked state, the unlocking operation for returning the rear blade group to the folded position thereof is performed after the shutter release operation and in the course of the returning travel of a cocking lever or an electromagnetic holding lever since the rear blade group must be returned to the folded position thereof at latest before the start of the travelling of the front blade group. Upon generation of a release signal by the shutter release operation, electromagnets for retaining the front blade group and the rear blade group at the unfolded position and the folded position thereof respectively are energized, and the front blade group and the rear blade group are once attracted to said electromagnets and then start the operations to open and close the exposure aperture in a predetermined time sequence.
However, the conventional focal plane shutter described above has a defect that said focal plane shutter cannot be used for high-speed successive photographing since the shutter allows a long time lag as measured from the generation of the shutter release signal to actual exposure start due to the time required until a current of a constant level flows to the electromagnets after the energizing of said electromagnets by the shutter release operation (or the release signal). In addition, the conventional focal plane shutter has another defect that the front blade group starts travelling before disappearance of the bound of the rear blade group which is caused when the rear blade group returns to the folded position thereof and stops at this position, thereby making the sequential operations very unstable.